As defined in the 1973 Book of ASTM Standards, part 16 on wood; adhesives, pp 487-8, particle board is defined as the generic term for a panel (or sheet) manufactured from lignocellulosic materials (usually wood) primarily in the form of discrete pieces or particles, as distinguished from fibers, combined with a synthetic resin binder and bonded together under heat and pressure in a hot-press by a process in which the entire interparticle bond is created by the added binder. The wood particles may be rather large chips or flakes, or small particles such as wood flour or sawdust.
The particle board industry was made possible after World War II as a result of the availability and use of synthetic thermosetting resins, such as urea-formaldehyde, phenolformaldehyde, malamine-formaldehyde and mixtures thereof. These synthetic resins are essential to the production of particle board since particles of wood lack much of the natural interfiber bonding that is possible in both paper and fiberboard.
Particle board is generally produced in flat, multiplaten presses to which a mixture of wood particles and synthetic resin is charged and subjected to fairly high temperatures and pressures for relatively short periods of time. Prior to compression molding in the multi-platen press, the mixture of wood particles and resin may be subjected to the action of a hammermill or attrition mill to reduce the size of the wood particles and to provide an intimate mixture of wood particles and resin. The high temperatures and pressures of the process are essential for the proper curing of the resin and, therefore, satisfactory bonding of the wood particles in the board.
Although particle board is generally produced in large presses, shaped articles can also be produced in smaller molds of specific design using wood particles in combination with synthetic resins. Temperatures, pressures and heating times are similar to those employed with the larger presses as dictated by the strength requirements of the end use.
Particle board is classified on a density basis, which is determined by the amount of pressure applied to the wood particles. Low density board (0.25-0.40 g/cm.sup.3) is generally used as panel material for heat and sound insulation. Medium density board (0.40-0.80 g/cm.sup.3) is the most commonly produced variety of particle board, while high density board (0.80-1.20 g/cm.sup.3) is very similar to conventional hardboard. Both medium and high density particle board are used in furniture construction and in a variety of molded articles such as core plugs, wall plaques, toilet seats and lazy susan trays, to mention a few.
Because of the scarcity and high cost of synthetic resins in recent years, a number of industries have added fillers or extenders such as cereal flours, starches, clays, etc. This practice has generally not been possible in the particle board industry because of the essential role of the synthetic resin in promoting the entire interparticle bond. Common fillers or extenders would only dilute the effectiveness of the resin, and, therefore, reduce the strength of the particle board.
In recent years a product has become available which is produced from gelatinized corn flour and a nitrogen containing compound according to procedures described in U.S. Pat. No. 3,725,324. This product has been used as a binder in drywall taping compounds. We have found that this product, as well as other amylaceous materials produced according to U.S. Pat. No. 3,725,324, can function as resin extenders in producing particle board while retaining the original strength properties of the board. This is a most surprising and unique finding in that most common amylaceous extenders have little or no positive effect on the function properties of the resin or binder with which they are used and only dilute the effect of the binder. However, we believe that the nitrogen containing compound and the gelatinization process used to produce the above described binder, and related amylaceous materials, have a pronounced effects on the properties of the resulting products.
For one thing, the gelatinization process tends to solubilize the starch, making more starch molecules available for interaction or actual chemical bonding. The nitrogen containing compounds are, under certain conditions, solvents for starch and may aid in making starch molecules available for reaction. The hydroxyl groups on the starch molecules offer a potential for reaction with the resin to form permanent covalent or ionic linkages with the resin. Thus the amylaceous material may become a permanent and functional part of the total binder system.
It is known that the nitrogen containing compounds modify the flow characteristics of the amylaceous material. This may allow the amylaceous material to function in a manner similar to the synthetic resins when subjected to high temperatures and pressures of the manufacturing process. The nitrogen compound itself may also enter into a reaction with the resin and become a permanent part of the total binder system. Thus, the amylaceous material of this invention may be functioning more like a resin than an extender in this application.
In addition to retaining strength properties of the board, the binder of U.S. Pat. No. 3,725,324 also retains the water absorption properties of the board, which are extremely important in certain applications. In certain resin systems the use of said binder has even eliminated the need to add materials, such as zinc stearate, which are used to promote good mold release during the manufacture of molded particle board pieces. In certain resin systems said binder of U.S. Pat. No. 3,725,324 has reduced or eliminated the odor of free formaldehyde which frequently accompanies the use of formaldehyde-based resins.
The before-mentioned binder and related amylaceous materials are much less expensive than synthetic resins. Therefore, the use of this binder as an extender of synthetic resins greatly reduces the cost of the total binder system used to produce particle board.